CIS 4360 Secure Computer Systems Symmetric Cryptography
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1 CIS 4360 Secure Computer Systems Symmetric Cryptography Professor Qiang Zeng Spring 2017
2 Previous Class Classical Cryptography Frequency analysis Never use home-made cryptography Goals of Cryptography Confidentiality, data integrity, authentication, nonrepudiation Building blocks of Cryptography Cryptographic hash, encryption, MAC, digital signature Cryptographic Hash Password storage, verifying data integrity Do not use MD5 and SHA-1 CIS 4360 Secure Computer Systems 2
3 Previous class By sending m and Hash(m), can the receiver verify the integrity of m, i.e., to verify whether m has been manipulated during the transmission? If not, what should you do? No, cryptographic hash can be used to verify data integrity only if the integrity of the hash value itself is assured. Typically, if the message and the digest value are transmiaed in the same channel, it is in vain as the adversary can replace both the message and the corresponding hash value. Message AuthenDcaDon Code and Digital Signature are used when the hash value can also be aaacked (discussed later) CIS 5512 Operating Systems 3
4 Cryptography Primitives Cryptography PrimiDves Symmetric Cryptography Cryptographic Hash Asymmetric Cryptography Symmetric EncrypDon Message AuthenDcaDon Code Asymmetric EncrypDon (encrypdon using PU; decrypdon using PR) Digital Signature (sign using PR; verify using PU) Stream Cipher Block Cipher CIS 4360 Secure Computer Systems 4
5 Notation C: ciphertext P: plaintext K: key PR: private key PU: public key E: encryption; e.g., C = E (K, P) D: decryption; e.g., P = D(K, C) H: hash : e.g., x y means x concatenated with y CIS 4360 Secure Computer Systems 5
6 Symmetric Block Cipher - DES DES: Data Encryption Standard NIST symmetric encryption standard Already broken; 56-bit key size is too small Block size: 64 bits 64-bit plaintext input; 64-bit ciphertext output How it works? Feistel structure: input block is divided into halves and processed alternatively XOR, Substitution (using the S-box), and Permutation (using the P-box) CIS 4360 Secure Computer Systems 6
7 Feistel Structure CIS 4360 Secure Computer Systems 7
8 Symmetric Block Cipher Triple-DES Triple-DES: still widely used C 1 = E(K 1, P), C 2 = D(K 2, C 1 ), C 3 = E(K 3, C 2 ) Key size: 3 x 56 = 168 bits; secure When k 1!= k 2 and k 1 = k 3 Key size: 2 x 56 = 112 bits; insecure When k 1 = k 2 = k 3, 3-DES becomes DES, since the second operation (D) offsets the first (E) It provides compatibility with DES when needed Disadvantage: slow CIS 4360 Secure Computer Systems 8
9 Symmetric Block Cipher - AES Advanced Encryption Standard (or Rijndael) Superseded DES as the NIST symmetric encryption standard since 2001 Block size: 128 bits (16 bytes) Key size: 128, 192 or 256 bits (slower with longer keys) DES vs. AES Longer key size -> more secure AES is faster than DES AES is suitable for parallel processing Encryption based on substitution and permutation CIS 4360 Secure Computer Systems 9
10 Serious Issue with Block Ciphers DES can only process 64-bit blocks AES can only process 128-bit blocks If you simply divide a long message into 64-bit or 128-bit blocks, and process them independently (this strategy is called Electronic Codebook ) Identical plaintext blocks lead the same ciphertext blocks CIS 4360 Secure Computer Systems 10
11 Serious Issue with Block Ciphers The simple strategy Electronic Codebook (ECB) leaks too much information By applying ECB encryption to the left bitmap image, you get the right one *- ECB CIS 4360 Secure Computer Systems 11
12 Modes of Operation for Block Ciphers A Mode of Operation describes how block ciphers are applied to a message longer than a block; usually, it is simply called Mode E.g., ECB (do not use it), CBC (Cipher Block Chaining), CFB (Cipher Feedback) How to interpret AES128-CBC? AES128: cipher block with 128-bit key CBC: mode of operation Similarly, you can interpret 3DES-CFB CIS 4360 Secure Computer Systems 12
13 CBC (Cipher Block Chaining) Even identical plaintext blocks will produce different ciphertext (as long as their preceding ciphertext blocks are different) Initialization Vector (IV) has the same size as the plaintext; by varying the IV, even if the same key is applied to encrypting two identical messages, the ciphertext is different Thus the adversary cannot infer whether the two messages are identical CIS 4360 Secure Computer Systems 13
14 Effect of Applying CBC *- CBC CIS 4360 Secure Computer Systems 14
15 Decryption based on CBC If you provide an incorrect IV, will you get wrong decryption results? Only the first block is corrupted; you still get correct results for other blocks This property is NOT necessarily true with other modes of operations CIS 4360 Secure Computer Systems 15
16 Stream Cipher - RC4 In stream ciphers, the key is used to generate a keystream (a key-seeded pseudorandom stream of bits), which is XORed with the plaintext CIS 4360 Secure Computer Systems 16
17 Stream Cipher - RC4 Rivest Cipher 4 (considered insecure) Key size: bits Warning: The first few bytes of the keystream leak the information of the key; so discard the first, say 1024, bytes of the keystream before using it CIS 4360 Secure Computer Systems 17
18 Block Cipher vs. Stream Cipher Stream Cipher is faster Stream Cipher is simpler to implement in h/w Stream Cipher can operate on a single bit In streaming, it is beneficial for reducing the latency Stream Cipher works well even if the length of the plaintext is unknown Stream Cipher does not need mode of operation Stream Cipher does not need padding (e.g., 120-bit data block is padded to 128 bits) CIS 4360 Secure Computer Systems 18
19 Block Cipher vs. Stream Cipher Stream Ciphers have security problems that Block Ciphers do not have Key cannot be reused C 1 = P 1 XOR K; C 2 = P 2 XOR K; C 1 XOR C 2 = (P 1 XOR K) XOR (P 2 XOR K) = P 1 XOR P 2 Bit-flipping attack Assume P 1 = 1000, P 2 = 9999, C 1 = P 1 XOR K You can get C 2 = P 2 XOR K by C 1 XOR P 1 XOR P 2 = (P 1 XOR K) XOR P 1 XOR P 2 = P 2 XOR K CIS 4360 Secure Computer Systems 19
20 Previous class By sending m and Hash(m) on Internet, can the receiver verify the integrity of m, i.e., to verify whether m has been manipulated during the transmission? If not, what should you do? No, cryptographic hash can be used to verify data integrity only if the integrity of the hash value itself is assured. Typically, if the message and the digest value are transmiaed in the same channel, it is in vain as the adversary can replace both the message and the corresponding hash value. Message AuthenDcaDon Code and Digital Signature are used when the hash value can also be aaacked (discussed later) CIS 5512 Operating Systems 20
21 MAC (Message Authentication Code) The digest h = H(m) is generated without any key, such that anyone (including the adversary) can create it What if a key is used? Now the adversary cannot forge a keyed-hash value without the key This is the purpose of MAC A MAC is a short string used to verify the message integrity and authentication mac = MAC(k, m) To prevent the replay attack, the message should contain timestamp, sequence number CIS 4360 Secure Computer Systems 21
22 A Popular MAC Algorithm: HMAC Hash-based Message Authentication Code A typical wrong design:mac(k, m) = H(k m) Susceptible to length extension attacks if the hash uses the Merkle Damgård Construction Based on H(k m) the adversary can extend the message and still get a valid MAC: H(k m m ) CIS 4360 Secure Computer Systems 22
23 A Popular MAC Algorithm: HMAC To resist Length Extension Attack HMAC(m, k) = H( k H( k m) ) // nested hash The hash function uses MD5, SHA-1, SHA-2 MD5 and SHA-1 are insecure Only SHA-2 is recommended now E.g., if SHA256 is used, it is called HMAC-SHA256 The key size >= the block size to make full use of the hash resistance In HMAC-SHA256, the key size should be 256 bit CIS 4360 Secure Computer Systems 23
24 KMAC SHA-3 Derived MAC SHA-3 does not use the Merkle-Damgård Construction, so is not vulnerable to the Length Extension Attack Can still use SHA-3 in HMAC (the nested hash), but it is over-kill KMAC (Keccak MAC) H(k m) Uses SHA-3 (Keccak) Simpler and faster than HMAC; Arbitrary-length output NIST in CIS 4360 Secure Computer Systems 24
25 Summary Cryptography PrimiDves Symmetric Cryptography Cryptographic Hash Asymmetric Cryptography Symmetric EncrypDon Message AuthenDcaDon Code Asymmetric EncrypDon (encrypdon using PU; decrypdon using PR) Digital Signature (sign using PR; verify using PU) Stream Cipher Block Cipher CIS 4360 Secure Computer Systems 25
26 Writing Assignments When to use Stream Ciphers? Can MAC be used to achieve non-repudiation? CIS 4360 Secure Computer Systems 26
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